Sustainability was being practiced in Boulder, Colorado’s water system long before sustainability was a trendy concept. There has been hydroelectric generation on Boulder’s water supply since 1910, and hydroelectric generation in the water treatment plants since the 1980s. Cogeneration at the wastewater treatment plant has been going on since the 1980s.

Of course, Boulder’s water source is high in the mountains, making hydro power feasible.

But Boulder also adopts sustainable practices not related to its geography. One recent project is the use of weak wort, a brewery waste product, to reduce nitrate in wastewater effluent. The weak wort is added during an anoxic phase of treatment.

“When oxygen is not present but nitrate is present, a group of organisms called heterotrophs essentially breathe the nitrate, and they consume readily biodegradable carbon,” says Cole Sigmon, engineering project manager for wastewater. “An issue with many water resource recovery facilities is that there’s plenty of nitrate for them to breathe but not enough carbon for them to eat to get the nitrate down to a level that’s being targeted. One strategy is to add an external source of food to drive the reaction.”

Making choices

The Boulder Water Resource Recovery Facility (25 mgd design, 12 mgd average) has a daily nitrate permit limit of 17.9 mg/L. “To meet that limit, we need an external source of carbon,” Sigmon says. The city contacted breweries and pharmaceutical and food manufacturers and was considering using methanol or acetic acid.

Methanol was rejected for safety reasons and the need for a special group of organisms to convert it to readily biodegradable carbon. Acetic acid was selected as an option, along with weak wort from the Avery Brewing, 3 miles from the plant. “Weak wort was very promising,” Sigmon says. “It has a really high fraction of readily biodegradable to slowly biodegradable carbon.”

At the time, Avery Brewing was constructing a new brewery, so it was fairly simple to set up a system to separate the weak wort from the wastewater stream. The new brewery needed an industrial pretreatment permit. “We worked with them on the permit,” Sigmon says. “Because the city was interested in the weak wort, we gave them an exemption so they could discharge it without the normal BOD surcharge.”

The city built a tank for weak wort at the brewery. “They control infrastructure that sends weak wort into the tank, and we control the infrastructure that stores weak wort and transfers it out of the tank,” Sigmon says. The city pays a third party to haul the weak wort to the treatment plant. The process began in February 2017. The city’s cost for the overall project was about $4.5 million; the initial pilot project received more than $1 million in grants.

Product testing

The carbon content of the weak wort, essentially sugar water, varies significantly with the type of beer. Wort from wheat beers has less sugar, while IPA wort has more. “We’ve seen an average of 60,000 mg/L of floc-filtered COD,” Sigmon says. “That’s a test that approximates readily biodegradable chemical oxygen demand, or what we call candy carbon, the carbon that we really want. We’ve seen loads as low as 17,000 and loads as high as almost 450,000.

“With every load, our operators take a sample to our lab and get a number to put into our SCADA system. That concentration tells the pumps where to start when they are dosing carbon for nitrate removal. Nitrate sensors in each anoxic zone are used for feedback control on the chemical dosing, so the pumps kick on when the nitrate concentration reaches a setpoint. That way the product is efficiently used.”

If the weak wort supply runs out, operators switch to acetic acid. Using weak wort is more cost-effective as long as it tests to at least 17,000 mg/L. The average is 60,000 mg/L. Avery Brewing plans to increase production under a partnership with another brewery. “Brewery waste directly offsets the amount of acetic acid we have to add, so we are excited that Avery is ramping up its brewing,” Sigmon says. There are some light suspended solids in the weak wort, but most appear to be consumed in the treatment process.

“Some are beneficially used for denitrification and some for energy production,” Sigmon says. “Between the anoxic zone and the aerated zone that follows it, we believe the majority of the product is consumed. We didn’t see a significant change in our solids production when we began adding carbon.”

Community partnership

Boulder has another partnership with a business for its Biogas Use Enhancement Project. Boulder’s cogeneration system is at the end of its life span, and replacing it would not be cost-effective. “Current practice is not financially viable,” Sigmon says. “The electricity and natural gas savings do not match the resources that we put into them.

“The economically viable thing to do, and what you are seeing a lot of Colorado utilities move toward, is production of natural gas for use as vehicle fuel. We plan to partner with Western Disposal. Our reclaimed natural gas will fuel 12 to 15 or so of its trucks.”  

The project is expected to begin production in 2020. The raw biogas will be scrubbed for removal of hydrogen sulfide, moisture removal, siloxanes, VOCs and carbon dioxide.

“After that, we’ll have natural gas of pipeline quality,” Sigmon says. The city is working with Public Service Co. of Colorado on an interconnect to deliver the finished gas to the grid.

Climate action

The project will have a very short payback and will fund some infrastructure upgrades already planned. “The sale of the reclaimed natural gas as vehicle fuel makes the city eligible for revenues available via EPA’s Renewable Fuel Standard,” Sigmon says. “Keeping this transaction and associated greenhouse gas emissions reductions within the community is important to us because of the city’s Climate Commitment.”

In 2016, the City Council unanimously approved the Climate Commitment, a strategy for transitioning to a clean-energy economy. Goals include powering the city with 100 percent renewable electricity by 2030 and reducing greenhouse gas emissions by at least 80 percent below 2005 levels by 2050.

Keys to the plan are developing renewable local energy sources, wise use of resources, enhancing urban and rural ecosystems, and fostering community climate action. The brewery waste and biogas projects help the city check a lot of boxes.